Methods of Receiving and Receivers

ABSTRACT

A receiver uses a local oscillator to receive data transmitted via a combination of radio frequency signals using carrier aggregation. Each radio frequency signal occupies a respective radio frequency band and the radio frequency bands are arranged in two groups, a first group and a second group, separated in frequency by a first frequency region, each of the groups including one or more radio frequency bands and the first group occupying a wider frequency region than the second group. The radio frequency signals are processed using the local oscillator by setting the local oscillator, during the processing, to a frequency that is offset from the centre of a band defined by outer edges of the frequency regions occupied by the two groups.

TECHNICAL FIELD

The present invention relates to methods of receiving and receivers forradio communication systems, and in particular, but not exclusively, tonon-contiguous carrier aggregation schemes.

BACKGROUND

Long Term Evolution (LTE) Advanced is a mobile telecommunicationstandard proposed by the 3^(rd) Generation Partnership Project (3GPP)and first standardised in 3GPP Release 10. In order to provide the peakbandwidth requirements of a 4^(th) Generation system as defined by theInternational Telecommunication Union Radiocommunication (ITU-R) Sector,while maintaining compatibility with legacy mobile communicationequipment, LTE Advanced proposes the aggregation of multiple carriersignals in order to provide a higher aggregate bandwidth than would beavailable if transmitting via a single carrier signal. This technique ofCarrier Aggregation (CA) requires each utilised carrier signal to bedemodulated at the receiver, whereafter the message data from each ofthe signals can be combined in order to reconstruct the original data.Carrier Aggregation can be used also in other radio communicationprotocols such as High Speed Packet Access (HSPA).

Carrier signals are typically composed of a carrier frequency that ismodulated to occupy a respective radio frequency carrier signal band.Contiguous Carrier Aggregation involves aggregation of carrier signalsthat occupy contiguous radio frequency carrier signal bands. Contiguousradio frequency carrier signal bands may be separated by guard bands,which are small unused sections of the frequency spectrum designed toimprove the ease with which individual signals can be selected byfilters at the receiver by reducing the likelihood of interferencebetween signals transmitted in adjacent bands. Non-contiguous CarrierAggregation comprises aggregation of carrier signals that occupynon-contiguous radio frequency carrier signal bands, and may compriseaggregation of clusters of one or more contiguous carrier signals. Thenon-contiguous radio frequency carrier signal bands are typicallyseparated by a frequency region which is not available to the operatorof the network comprising the carrier signals, and may be allocated toanother operator. This situation is potentially problematic for thereception of the carrier signals, since there may be signals in thefrequency region that separates the non-contiguous carriers which are ata higher power level than the wanted carrier signals.

A Direct Conversion Receiver (DCR) is typically employed to receivecellular radio signals, and typically provides an economical and powerefficient implementation of a receiver. A DCR uses a local oscillatorplaced within the radio frequency bandwidth occupied by the signals tobe received to directly convert the signals to baseband. Signals on thehigh side of the local oscillator are mixed to the same basebandfrequency band as signals on the low side of the local oscillator, andin order to separate out the high and low side signals, it is necessaryto mix the signal with two components of the local oscillator inquadrature (i.e. 90 degrees out of phase with one another) to produceinphase (I) and quadrature (Q) signal components at baseband. The I andQ components are digitised separately, and may be processed digitally toreconstruct the separate high side and low side signals. Thereconstructed high and low side signals may be filtered in the digitaldomain to separate carrier signals received within the receiverbandwidth of the DCR.

The presence of a higher power signal in the region separatingnon-contiguous carrier clusters poses particular problems if a DCR is tobe used to receive a band of frequencies comprising non-contiguousCarrier Aggregation signals. In particular, since the higher powersignal is within the receiver bandwidth, the dynamic range of thereceiver need to encompass the powers of the wanted carrier signals,which are typically received at a similar power to each other, and thehigher power signal. This may place severe demands on the dynamic rangeof the analogue to digital converter (A/D) in particular. Furthermore,due to inevitable imbalances between the amplitudes and phases of the Iand Q channels, the process of reconstructing the separate high side andlow side signals suffers from a limited degree of cancellation of theimage component; that is to say, some of the high side signals breakthrough onto the reconstructed low side signals, and vice versa. Thedegree of rejection of the image signal may be termed the Image RejectRatio (IRR). If the higher power signal is a high side signal, it maycause interference to received low side signals due to the finite IIR,and similarly if the higher power signal is a low side signal, it maycause interference to received high side signals

One conventional method of receiving Non-contiguous Carrier Aggregationsignals is to provide two DCR receiver stages, each having a localoscillator tuned to receive a cluster of contiguous carriers, and sorejecting signals in the frequency region between the clusters beforedigitisation. However, this approach is potentially expensive and powerconsuming, and may suffer from interference between the closely spacedlocal oscillators.

It is an object of the invention to address at least some of thelimitations of the prior art systems.

SUMMARY

In accordance with a first exemplary embodiment of the presentinvention, there is provided a method of receiving, using a localoscillator, data transmitted via a combination of at least a pluralityof radio frequency signals using carrier aggregation, the methodcomprising:

processing a at least said plurality of radio frequency signals usingsaid local oscillator, each radio frequency signal occupying arespective band of a plurality of radio frequency bands, the pluralityof radio frequency bands being arranged in two groups, a first group anda second group, separated in frequency by a first frequency region, eachof the groups including one or more radio frequency bands and the firstgroup occupying a wider frequency region than the second group; and

setting said local oscillator, during said processing, to a frequencythat is offset from the centre of a band defined by outer edges of thefrequency regions occupied by the two groups.

In accordance with a second exemplary embodiment of the presentinvention, there is provided a receiver for receiving data transmittedvia a combination of at least a plurality of radio frequency signalsusing carrier aggregation, each radio frequency signal occupying arespective band of a plurality of radio frequency bands, the pluralityof radio frequency bands being arranged in two groups separated infrequency by a first frequency region, the first of the two groupsoccupying a wider frequency region than the second group, the receivercomprising:

a controller configured to determine a frequency that is offset from thecentre of a band defined by outer edges of the frequency regionsoccupied by the two groups; and

a signal processor for processing said plurality of radio frequencysignals using a local oscillator set to the determined frequency.

Further features and advantages of the invention will be apparent fromthe following description of preferred embodiments of the invention,which are given by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the transmission of carrieraggregation signals by the radio access network of a first operator andtransmission of a signal from another a radio access network;

FIG. 2 is amplitude-frequency diagram showing carriers in anon-contiguous carrier aggregation method and a carrier from anotheroperator received at a higher level;

FIG. 3 is a schematic diagram showing a conventional direct conversionreceiver;

FIG. 4 is a diagram illustrating an effect of a finite image rejectionratio in a direct conversion receiver;

FIG. 5 is a diagram illustrating reception of non-contiguous aggregatedcarriers in a low IF receiver.

FIG. 6 is a schematic diagram showing a conventional low IF receiver;

FIG. 7 is a diagram illustrating problems with reception ofnon-contiguous aggregated carriers in a direct conversion receiver;

FIG. 8 is an amplitude-frequency diagram illustrating reception ofnon-contiguous aggregated carriers in a direct conversion receiver in anembodiment of the invention;

FIG. 9 is an amplitude-frequency diagram illustrating reception ofnon-contiguous aggregated carriers in an receiver having differentpassband filters for the high side and low side signals in an embodimentof the invention;

FIG. 10 is a schematic diagram showing a receiver having two zero IFbranches each having different bandpass filters in an embodiment of theinvention;

FIG. 11 is a schematic diagram showing an alternative receiver havingtwo zero IF branches each having different bandpass filters in anembodiment of the invention;

FIG. 12 is a diagram illustrating a conventional direct conversionreceiver as implemented in an RFIC;

FIG. 13 is a frequency-amplitude diagram illustrating problems withreception of non-contiguous aggregated carriers in a direct conversionreceiver, showing image frequencies at the equivalent position in RFfrequency;

FIG. 14 is a frequency-amplitude diagram illustrating a conventionalsolution for the reception of non-contiguous aggregated carriers, by theuse of two receivers, each having a different local oscillatorfrequency;

FIG. 15 is a schematic diagram illustrating an RFIC implementation forthe reception of non-contiguous aggregated carriers, by the use of tworeceivers, each having a separate RFIC and a different local oscillatorfrequency;

FIG. 16 is a schematic diagram illustrating an RFIC implementation forthe reception of non-contiguous aggregated carriers, by the use of tworeceivers, each having a different local oscillator frequency on asingle RFIC;

FIG. 17 is an amplitude-frequency diagram illustrating reception ofnon-contiguous aggregated carriers, with a single signal from anotheroperator between the wanted carrier signals in an embodiment of theinvention;

FIG. 18 is an amplitude-frequency diagram illustrating the reception ofnon-contiguous aggregated carriers, showing a single signal from anotheroperator between carrier aggregation clusters and the effect of imagefrequencies in an embodiment of the invention;

FIG. 19 is an amplitude-frequency diagram illustrating the reception ofnon-contiguous aggregated carriers, showing two signals from anotheroperator between carrier aggregation clusters and the effect of imagefrequencies in an embodiment of the invention;

FIG. 20 is an amplitude-frequency diagram illustrating the reception ofnon-contiguous aggregated carriers, showing three signals from anotheroperator between carrier aggregation clusters and the effect of imagefrequencies in an embodiment of the invention;

FIG. 21 is an amplitude-frequency diagram illustrating the reception ofnon-contiguous aggregated carriers, showing a different filter bandwidthused for the reception of high side and low side signals in anembodiment of the invention;

FIG. 22 is an amplitude-frequency diagram illustrating the reception ofnon-contiguous aggregated carriers, showing a) the use of a complexfilter characteristic b) the effect of the complex filter characteristicshown with a digital filter characteristic superimposed and c) thecombined effect of the complex and digital filters;

FIG. 23 (upper part) is schematic diagram showing a receiverarchitecture having complex filters and a digital data path; and

FIG. 23 (lower part) is schematic diagram showing a receiverarchitecture having real filters and a digital data path having imagereject mixing.

DETAILED DESCRIPTION

By way of example an embodiment of the invention will now be describedin the context of a wireless communications system supportingcommunication using E-UTRA radio access technology, as associated withE-UTRAN radio access networks in LTE systems. However, it will beunderstood that this is by way of example only and that otherembodiments may involve wireless networks using other radio accesstechnologies, such as UTRAN, GERAN or IEEE802.16 WiMax systems.

FIG. 1 shows the transmission of radio frequency signal signals 10 a, 10b and 10 c by the radio access network to a receiver 8. The radiofrequency signals each occupy a respective carrier signal band, as shownin the amplitude-frequency diagram of FIG. 2. A carrier signal band isthe part of the radio frequency spectrum occupied by a modulated radiofrequency carrier comprising the radio frequency signal. Radio frequencysignals 10 a, 10 b, and 10 c occupy radio frequency bands 14 a, 14 b and14 c as shown in FIG. 2. Data is received using the combination of theradio frequency signals 10 a, 10 b and 10 c, and the bands 14 a, 14 band 14 c shown in FIG. 2 represent a set of radio frequency signals,that may be referred to as component carriers, transmitted using CarrierAggregation. It can be seen from FIG. 2 that non-contiguous CarrierAggregation is used, since a radio frequency signal from anotheroperator, other than the operator sending the data, is present in afrequency region separating bands 14 b and 14 c. In FIG. 1, the radiofrequency signals are sent from a first base station 4, operated byOperator A. A second base station 6, operated by a different operator,Operator B, is situated within the area of coverage 2 of the first basestation 4, and transmits a radio frequency signal 12 that is receive bythe user equipment 8. It can be seen that the second base station iscloser to the user equipment 8 than is the first base station. As aresult, it can be seen from FIG. 2 that the radio frequency signal isreceived at the user equipment 8 at a significantly higher power level,as shown by the amplitude of the band 16 transmitted by operator B.

FIG. 3 is a schematic diagram showing a conventional direct conversionreceiver. A signal is received by an antenna 100, and filtered by afront end filter 102, which removes out of band signals, protecting theLow Noise Amplifier (LNA) 104 from saturation by strong out of bandsignals. A local oscillator 106 is typically set to a frequency in thecentre of a desired radio frequency (RF) band. RF signals that are bothhigher than (high side) and lower than (low side) the local oscillatorfrequency are mixed with the local oscillator to downconvert the RFsignals to baseband frequencies, which are the difference between the RFand local oscillator frequencies. These difference frequencies, forsignals within an intended receive band, are arranged to fall within thepassband of the low pass filters 114, 116 of the direct conversionreceiver. In order to distinguish between RF signals that originated onthe high side of the local oscillator and RF signals that originated onthe high side of the local oscillator, it is necessary to mix the RFsignal with two components of the local oscillator which are inquadrature (i.e. 90 degrees out of phase with one another) to produceinphase (I) and quadrature (Q) signal components at baseband. As shownin FIG. 3, the local oscillator is split into 0 and −90 degreecomponents in a splitter 108 and each component is mixed with theincoming RF signal in a respective mixer 110, 112. The I and Qcomponents are separately filtered low pass filtered, and each filteredsignal is converted to the digital domain in an Analogue to digitalconverter (A/D) 118, 120, to produce a data stream with I and Qcomponents 122, 124. The I and Q components may be processed digitallyto reconstruct the separate high side and low side signals. Thereconstructed high and low side signals may be filtered in the digitaldomain to separate carrier signals received within the receiverbandwidth of the DCR. However, as already mentioned, due to imbalancesbetween the amplitudes and phases of the I and Q channels, the processof reconstructing the separate high side and low side signals suffersfrom a limited degree of cancellation of the image component, so thatsome of the high side signals break through onto the reconstructed lowside signals, and vice versa. The degree of rejection of the imagesignal may be termed the Image Reject Ratio (IRR).

FIG. 4 shows the effect of a finite image rejection ratio in a directconversion receiver, in the case where two bands 202, 204 are receivedat approximately the same power level at radio frequency. As can beseen, the two bands are mixed with a local oscillator 206 anddownconverted to a band encompassing zero frequency, which may bereferred to as DC (Direct Current). In FIG. 4, the high side signal 204is shown as being downconverted to positive frequency 210, and the lowside signal 202 is shown as being downconverted to a negative frequency208. This is a matter of convention, and the designation of positive andnegative frequencies may be transposed. The concept of positive andnegative frequencies has meaning only within the complex signal domain,in which signals are represented by I and Q components. A negativefrequency has a phasor defined by its I and Q components that rotates inthe opposite direction to the phasor of a positive frequency. Bydistinguishing between positive and negative frequencies by signalprocessing, for example using a Fast Fourier Transform (FFT) or acomplex digital mixer, signals originating as high side RF signals maybe separately received from signals originating as low side RF signals.So, as shown in FIG. 4, data may be extracted from two received carriersignal bands, provided that the signal to noise ratio (SNR) is notdegraded unacceptably by the image component 214 of the high side signal204 that is in the same band 208 as the downconverted low side signal202, and the image component 212 of the low side signal 202 that is inthe same band 210 as the downconverted high side signal 204. For signalsreceived at approximately the same power level, SNR is not usuallydegraded unacceptably by the finite image reject ratio.

FIG. 5 is a diagram illustrating reception of non-contiguous aggregatedcarriers. In this example, wanted component signal bands 302 and 302 areseparated by a higher power radio frequency signal 318, which mayoriginate from another operator. As can be seen from FIG. 5, a localoscillator 306 may be placed in the middle of a receive band defined bythe three component signal bands 302, 304, 318. As can be seen from FIG.5, images of the higher power radio frequency signal resulting from thefinite image reject ratio do not fall on top of the downconverted weakersignals in this case, but fall within the downconverted components 320of the higher power radio frequency signal.

FIG. 6 is a schematic diagram showing a conventional low IF receiverthat may be used to receive the signals illustrated in FIG. 5. It can beseen that the low IF receiver differs from a conventional DCR receiverin that the low pass filters of a conventional DCR receiver, as shown inFIG. 3, have been replaced by bandpass filters 114, 118, to filter the Iand Q signals respectively. The band pass characteristics of the bandpass filters have been shown on FIG. 5, as the dashed lines 324, 322,around the wanted component signal bands 308, 322. It can be seen thatthe downconverted components 320 of the higher power radio frequencysignal are rejected by the band pass filters in the I and Q signalpaths, so that saturation of the A/D converter by the unfiltereddownconverted components 320 of the higher power radio frequency signalmay be avoided.

FIG. 7 is a diagram illustrating problems with reception ofnon-contiguous aggregated carriers in a direct conversion receiver. Thisillustrates the situation shown in FIG. 2, in which component signalsbands 524, 502, 504 in a non-contiguous carrier aggregation system arearranged in two groups, or clusters, the first group occupying a widerfrequency region than the second group. A higher power signal 518 islocated between in a frequency region between the first group and thesecond group. In this case, by contrast to the situation shown in FIG.5, it can be seen that the images 528 of the higher power radiofrequency signal that result from the finite image reject ratio falldirectly in the same band as one of the downconverted component signalbands 508. Depending on the difference between the received power of thehigher power signal, the received power of the wanted received signals,and the image reject ration, this situation may prevent reliabletransmission of the signals in band 508.

FIG. 8 shows a solution to the problems illustrated by FIG. 7 in anembodiment of the invention. As can be seen, the local oscillator isoffset from the centre of the band encompassing the wanted signals, thatis to say offset from the centre of the band 530 defined by acombination of the frequency regions occupied by the two groups ofsignals and the frequency region in between, i.e. offset from the centreof a band defined by outer edges of the frequency regions occupied bythe two groups.

When the LO frequency is set as shown in FIG. 8, it can be seen that theimages 528 of the higher power radio frequency signal that result fromthe finite image reject ratio is only partly overlapping thedownconverted component signal band 508. As can be seen, part of thebands are affected by the image while other parts are not. Due tointerleaving of subcarriers and the use of error correction coding, atypical modulation format, such as Orthogonal Frequency DivisionMultiplexing (OFDM), may be tolerant to the degradation of a proportionof the band, whereas it would not be tolerant if the degradation wereapplied to the whole band. Therefore, the situation in FIG. 8 may allowacceptable reception of component signal band 508, whereas the situationin FIG. 7 may not. As can be seen from FIG. 8, preferably the LO is setsuch that the distance from the LO to the centre of each of the twowanted clusters 532, 534 is equal. Setting the local oscillator in thisway has the advantage of minimising interference due to finite imagerejection ratio resulting from both an unwanted signal between thewanted signal clusters, and also minimising interference from unwantedsignals adjacent to the wanted signal clusters situated away from thelocal oscillator frequency. In an embodiment of the invention, theoffset of the local oscillator frequency may be determined in dependenceon a measurement of signal quality, such as signal to noise plusinterference ratio, of at least one of the plurality of radio frequencysignals. For example, if an unwanted signal adjacent to the wantedsignal clusters situated away from the local oscillator frequency on thehigh frequency side is greater than another unwanted signal adjacent tothe wanted signal clusters situated away from the local oscillatorfrequency on the low frequency side, it may be determined that the localoscillator offset should be set at a position that causes the leasttotal interference with the wanted signals. This may be determined onthe basis of signal to noise plus interference ratio measurements foreach of the wanted signals.

FIG. 9 shows that that setting of the local oscillator may be used inconjunction with a receiver having two bandpass filter characteristics540, 538 one of which 538 is wider than the other 540. The bandpasscharacteristics may be set to be appropriate to receive the componentsignal bands in the respective groups of signals.

FIG. 10 is a schematic diagram showing a receiver having two bandpassfilter characteristics as illustrated in FIG. 9 in an embodiment of theinvention. The receiver has two branches. A first branch is a low IFreceiver having I and Q channels, each of which has a bandpass filters814, 816 with a first bandwidth. A second branch is also configured as alow IF receiver as shown in FIG. 10, and also has I and Q channels, eachof which has a bandpass filters 814, 816 with a second bandwidth,different from the first bandwidth. A first subset of downconvertedradio signals may be received using the first branch, and a secondsubset of downconverted radio signals may be received using the secondbranch.

FIG. 11 is a schematic diagram showing an alternative receiver havingtwo branches each having different bandpass filters in an embodiment ofthe invention, in which a single set of quadrature mixers is sharedbetween the two branches.

Embodiments of the invention will now be described in more detail.Embodiments of the invention relate to multi-carrier wireless systems,using carrier aggregation. Operators may own non-contiguous allocationof spectrum; this may come about, for example, if an operator buysanother operator's businesses. If the spectrums happen to benon-adjacent then the allocation is non-contiguous. Operators typicallywish to exploit their spectrum as effectively as possible, so the needfor non-contiguous multi-carrier systems is increasing. An example ofsuch scenario is presented in FIG. 2. In a scenario such as thatillustrated in FIG. 2, there may be a problem with single receiver chainarchitecture in that it may not be known or guaranteed a priori what isallocated in the gap between the two non-contiguous carriers. Typically,another operator's licensed spectrum may be present in the gap.Furthermore, it cannot be guaranteed that the other operator's signal,that is to say deployed spectrum, is not significantly stronger than thewanted signal at the receiver input. This may place large demands on thereceiver performance in terms of dynamic range and image rejectionperformance.

Table 1 below gives example of possible allocations of blocks ofcarriers within a single band. In table 1, in the column headed“configuration”, “C” represents a 5 MHz component carrier and the gaplength is expressed as a number in MHz.

TABLE 1 Summary of operators' scenarios. Number of Gap ComponentScenario Band length Carriers Configuration A I 5 2 C-5-C B I 5 3 C-5-CCC I 10 4 C-10-CCC D IV 5 2 C-5-C E IV 10 3 C-10-CC F IV 15 4 CC-15-CC GIV 20 3 CC-20-C H IV 25 4 CC-25-CC

The reception of two or more non-contiguous component carriers causesseveral design challenges for a receiver containing one reception branchonly. The simplified block diagram of a typical direct-conversionreceiver (DCR) is presented in FIG. 12. The signal is amplified in thelow-noise amplifier (LNA) before being down-converted to zerointermediate frequency (IF). For phase- and frequency-modulated signals,the down-conversion must be performed with quadrature local oscillator(LO) signal to prevent signal sidebands from aliasing on one another.Prior to analogue-to-digital conversion (ADC or A/D), the signal islow-pass filtered and amplified such that the signal for the ADC is atsufficient level. A DCR is typically used in cellular user equipments(UEs) in, for example, GSM, WCDMA, HSPA, and single-carrier LTE modes,for example in Release 7, 8 or 9 LTE. From the point of view ofintegrated circuit development, DCR has several advantages compared toother receiver types, such as low complexity and power consumption,small silicon area, and a low number of off-chip components.

For a single receiver UE comprising conventional DRC hardware as shownin FIG. 12, the scenario shown in FIG. 2 is challenging. Firstly, sincedeployed spectrum of operator B shown is located in the wanted channel,is passes through the analogue circuitry without any filtering. Thus,the dynamic range of the analogue-to-digital converter (ADC) needs to beincreased by the amount of power difference between the unwanted andwanted carriers. In addition to the increased bandwidth required toreceive non-contiguous aggregated carriers, the dynamic rangerequirement makes ADC design even more challenging and power consuming.

Secondly, the gain control of the receiver becomes more challenging,since the maximum gain setup in different RF front-end blocks (LNA,Mixer, filters) is dominated by the strong unwanted carrier to preventthe receiver from saturation and/or clipping. As a result, the gain maybe set to a lower value than would be ideally required for the weakercarriers, thus deteriorating the signal-to-noise performance of theweaker carriers.

Thirdly, in practice, due to imperfections such as component mismatch indown-conversion mixers and analogue baseband filters and the quality ofquadrature signals from the local oscillator, there is a finiteamplitude and phase balance between the in-phase (I) and quadraturephase (Q) branches. That is to say, there are errors in matching betweenthe phase and amplitude of the inphase and quadrature signals paths. Ashas been already mentioned, this leads to a finite image reject ratio(HR).

FIG. 13 depicts a case, such as, for example, may result from 4 carrierHigh Speed Downlink Packet Access (4C-HSDPA) with strong unwantedcarrier received and down-converted with a demodulator having a finiteIQ performance. Due to the finite image-reject ratio (IRR), the morepowerful unwanted carrier will generate a strong image signaloverlapping the weaker carrier locating at opposite side of the LO. Thismay not achieve sufficient signal-to-noise ratio (SNR) to receive theweaker carrier.

So, as has been mentioned, the reception of non-contiguous CA signals ina conventional DCR receiver presents challenges regarding the ADC design(dynamic range vs. power consumption), RF/analogue gain control, and RFimages. These challenges apply to both the reception of non-contiguous(NC) carrier aggregation in HSDPA and LTE, and to the use ofnon-contiguous carrier aggregation for future standards to achieve highpeak data rates. Furthermore, high SNR figures are needed to be able tooperate with 64QAM modulation to reach the highest data rates. As aresult, a small impairment in signal quality or dynamic range caused bythe presence of the operator B signal can have a significant effect.

It is preferable that a single direct-conversion receiver is utilised inuser equipment intended to receive NC-HSDPA (or non-contiguous LTE), asthe user equipment may also be configured for lower data rates andsingle carrier operation, and user expectations would be for similar orbetter battery life than legacy UEs when operating at lower data rates(i.e. in non-carrier aggregation mode). However, as already mentioned, aUE with a conventional single receiver path is unlikely to be able toreceive intra-band non-contiguous carriers with maximal SNR.

One potential method of receiving non-contiguous carrier aggregationsignals is to receive separate clusters of component carriers inseparate receiver chains, each having a LO signal of its own. This isdepicted in FIG. 15, where Cluster1 and Cluster2 are each handled by aseparate respective receiver chain, as shown in FIG. 15. However, thesolution illustrated by FIG. 15 may increase the complexity of the FrontEnd Module (FEM), due to the need for signal splitting and the need tominimise local oscillator coupling between channels, which in turn maylead to a higher cost and increased insertion loss. In addition, in thesolution presented in FIG. 16, having two LO synthesizers operating atfrequencies close each other might suffer from LO pulling, which canlead to increased phase noise, instability and presence of sidebandtones. Within a single die it is challenging to achieve sufficientisolation between two LOs having a small frequency separation betweeneach other. Possibly, two simultaneously running synthesisers couldoperate at two completely different RF frequencies but the final LOfrequency could be generated with different frequency division ratios(e.g. 4 GHz divided by 2 and 6 GHz divided by 3). That solution,however, may lead to complicated design (either fractional or oddfrequency division ratios could be needed) and would possibly generateunwanted tones.

In an embodiment of the invention, a DCR is configured such that it isable to handle two non-contiguous clusters with improved SNR with asingle Radio Frequency Integrated Circuit (RFIC). In an embodiment ofthe invention, two clusters are each received with a different bandwidthfilter.

FIG. 17 presents a scenario similar to one shown in FIG. 13, except thefirst and second adjacent channels are now presented. In an embodimentof the invention, the LO signal is placed offset from the centre of theillustrated band to be received, as shown in FIG. 17. This has theadvantage that the effect of resulting images signals is minimized, asillustrated in FIG. 18. After the LO frequency is placed as shown inFIG. 18, the images of the unwanted adjacent channels are only partlyoverlapping with wanted channels in Cluster1 as shown. The average SNRimpairment across a band due to image signal folding, that is to say dueto finite image reject ratio, is thus reduced in the worst affectedbands at the expense of degrading the SNR impairment in bands that werenot affected with a conventional placing of the local oscillator. As canbe seen, part of the bands are affected by signal folding while otherparts are not. As has already been mentioned, due to interleaving ofsubcarriers and the use of error correction coding, a typical modulationformat, such as OFDM, may be tolerant to the degradation of a proportionof the band, whereas it would not be tolerant if the degradation wereapplied to the whole band.

An additional example is presented in FIG. 19. The scenario is similarto the previous one but now there are two carriers deployed by the otheroperator in the centre of the band, as shown in FIG. 19( a). Aconventional approach to the reception of the signals shown in FIG. 19(a) is shown in FIG. 19( b), in which the LO is placed between the twounwanted carriers, but as a result, one of the wanted carriers suffersfrom image signal due to the first adjacent high side channel. In anembodiment of the invention, this is mitigated by placing the LO suchthat the distance from the LO to the centre of each of the two wantedclusters is equal, as shown in FIG. 19 (c). As a result, afterdown-conversion the image of the wanted carrier in the narrow cluster islocated between the two wanted carriers, as shown in FIG. 19 (d). Now,image signals due to adjacent channels overlap the wanted carriers onlypartly and SNR degradation is averaged over the channel. As alreadymentioned, a typical modulation and coding format may be tolerant of areduced SNR over a part of the band.

FIG. 20 gives an example of a scenario in which there are three unwantedcarriers between the wanted clusters. As shown in FIGS. 20( a) and20(b), a conventional LO location may be at the centre of the mostpowerful carrier. Then, the image due this most powerful carrier wouldbe placed on top of the most powerful carrier itself, as shown in FIG.20( b). However, the SNR degradation due to image folding is minimizedin an embodiment of the invention, when the LO is placed substantiallyhalf way between the centres of the clusters, as shown in FIG. 10( c),or at least within approximately an eighth of a carrier bandwidth ofthis position.

In an embodiment of the invention, the improved positioning of the LOmay be used advantageously in combination with a low IF receiver. A lowIF receiver may be realised as illustrated in FIG. 6 by the substitutionof a band pass filter for the low pass filter of a conventional directconversion receiver.

FIGS. 20( a) and 20(c) show the passband filter characteristic of a lowIF receiver, shown referred to RF frequencies. As may be seen from acomparison of FIG. 20( a) with FIG. 20( c), the passband filter in thecase illustrated by FIG. 20( c) attenuates adjacent channels of Cluster2more efficiently than that in the case illustrated by FIG. 20( a).

In an embodiment of the invention, the improved positioning of the LOmay be used advantageously in combination with a low IF receiver, havingtwo receiver branches, one receiver branch having a different bandpassfilter characteristic from the other. Such a two-branch low IF receiveris shown in FIGS. 10, and an alternative implementation is shown in FIG.11. As shown in FIG. 21( b), the use of a narrower bandpass filter tofilter the narrower cluster, Cluster2, improves the rejection ofadjacent channels, as compared to the case with a the use of the samefilter bandit to receive high and low side signals, as in the case shownin FIG. 21( a). FIG. 21( a) may represent the case, for example, inwhich a single branch low IF receiver used.

The use of analogue bandpass filters may reduce the dynamic rangerequired by the A/D converter, since interfering signals may be removedbefore conversion.

In an embodiment of the invention, the analogue, typically bandpassfilters, are implemented using a complex filtering method, that is tosay each filter may process components of both the I and Q channels.Then, the filter response is asymmetric in respect to zero frequency asshown in FIG. 22( a). In this case, the image signal located at theopposite side of the zero frequency can be filtered out. As a result,carrier separation in the digital domain could be implemented withtypical digital down-conversion mixers as shown in the upper part ofFIG. 23. Alternatively, if conventional real-only analogue filters areused, the digital down-conversion could comprise a complex scheme toattenuate the image signal, as shown in the lower part of FIG. 23.

Although at least some aspects of the embodiments described herein withreference to the drawings comprise computer processes performed inprocessing systems or processors, the invention also extends to computerprograms, particularly computer programs on or in a carrier, adapted forputting the invention into practice. The program may be in the form ofnon-transitory source code, object code, a code intermediate source andobject code such as in partially compiled form, or in any othernon-transitory form suitable for use in the implementation of processesaccording to the invention. The carrier may be any entity or devicecapable of carrying the program. For example, the carrier may comprise astorage medium, such as a solid-state drive (SSD) or othersemiconductor-based RAM; a ROM, for example a CD ROM or a semiconductorROM; a magnetic recording medium, for example a floppy disk or harddisk; optical memory devices in general; etc.

It will be understood that the processor or processing system orcircuitry referred to herein may in practice be provided by a singlechip or integrated circuit or plural chips or integrated circuits,optionally provided as a chipset, an application-specific integratedcircuit (ASIC), field-programmable gate array (FPGA), etc. The chip orchips may comprise circuitry (as well as possibly firmware) forembodying at least one or more of a data processor or processors, adigital signal processor or processors, baseband circuitry and radiofrequency circuitry, which are configurable so as to operate inaccordance with the exemplary embodiments. In this regard, the exemplaryembodiments may be implemented at least in part by computer softwarestored in (non-transitory) memory and executable by the processor, or byhardware, or by a combination of tangibly stored software and hardware(and tangibly stored firmware).

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments. Furthermore, equivalents and modificationsnot described above may also be employed without departing from thescope of the invention, which is defined in the accompanying claims.

1. A method of receiving, using a local oscillator, data transmitted viaa combination of at least a plurality of radio frequency signals usingcarrier aggregation, the method comprising: processing a at least saidplurality of radio frequency signals using said local oscillator, eachradio frequency signal occupying a respective band of a plurality ofradio frequency bands, the plurality of radio frequency bands beingarranged in two groups, a first group and a second group, separated infrequency by a first frequency region, each of the groups including oneor more radio frequency bands and the first group occupying a widerfrequency region than the second group; and setting said localoscillator, during said processing, to a frequency that is offset fromthe centre of a band defined by outer edges of the frequency regionsoccupied by the two groups.
 2. A method according to claim 1, whereinthe first frequency region is not used for transmitting said data.
 3. Amethod according to claim 1, wherein the first frequency regioncomprises a radio frequency band occupied by a radio frequency signalthat is not aggregated with said plurality of radio frequency signals.4. A method according to claim 1, wherein at least one of the groupsincludes non-contiguous radio frequency bands.
 5. A method according toclaim 1, wherein the frequency to which the local oscillator is set iswithin one quarter of the bandwidth of one of the plurality of radiofrequency bands from a frequency mid-way between the centre of thefrequency region occupied by the first group and the centre of thefrequency region occupied by the second group.
 6. A method according toclaim 1, wherein the frequency to which the local oscillator is set issubstantially mid-way between the centre of the frequency regionoccupied by the first group and the centre of the frequency regionoccupied by the second group.
 7. A method according to claim 1, themethod comprising: determining the offset in dependence on a measurementof signal quality of at least one of the plurality of radio frequencysignals.
 8. A method according to claim 1, wherein said processingcomprises: downconverting said plurality of radio frequency signalsusing quadrature mixing to give inphase and quadrature components;filtering said inphase and quadrature components using a first bandpassfilter bandwidth to give first bandpass filtered inphase and quadraturecomponents; and filtering said inphase and quadrature components using asecond bandpass filter bandwidth, different from the first bandpassfilter bandwidth, to give second bandpass filtered inphase andquadrature components.
 9. A method according to claim 8, the methodcomprising using a complex filter to perform at least one of the stepsof: filtering said inphase and quadrature components to give firstbandpass filtered inphase and quadrature components using a firstcomplex filter; and filtering said inphase and quadrature components togive second bandpass filtered inphase and quadrature components using asecond complex filter.
 10. A method according to claim 7, furthercomprising receiving a first subset of the downconverted plurality ofradio frequency signals using the first bandpass filtered inphase andquadrature components and receiving a second subset of the downconvertedplurality of radio frequency signals using the second bandpass filteredinphase and quadrature components, wherein: the first subset of thedownconverted plurality radio frequency signals are downconverted fromradio frequency bands in the first group; and the second subset of thedownconverted plurality radio frequency signals are downconverted fromradio frequency bands in the second group.
 11. A receiver for receivingdata transmitted via a combination of at least a plurality of radiofrequency signals using carrier aggregation, each radio frequency signaloccupying a respective band of a plurality of radio frequency bands, theplurality of radio frequency bands being arranged in two groupsseparated in frequency by a first frequency region, the first of the twogroups occupying a wider frequency region than the second group, thereceiver comprising: a controller configured to determine a frequencythat is offset from the centre of a band defined by outer edges of thefrequency regions occupied by the two groups; and a signal processor forprocessing said plurality of radio frequency signals using a localoscillator set to the determined frequency.
 12. A receiver according toclaim 11, wherein the first frequency region is not used fortransmitting said data.
 13. A receiver according to claim 11, whereinthe first frequency region comprises a radio frequency band occupied bya radio frequency signal that is not aggregated with said plurality ofradio frequency signals.
 14. A receiver according to claim 11, whereinthe controller is configured to determine a frequency within one quarterof the bandwidth of one of the plurality of radio frequency bands from afrequency mid way between the centre of the frequency region occupied bythe first group and the centre of the frequency region occupied by thesecond group.
 15. A receiver according to claim 14, wherein thecontroller is configured to determine a frequency substantially mid waybetween the centre of the frequency region occupied by the first groupand the centre of the frequency region occupied by the second group. 16.A receiver according to claim 11, the method comprising: determining theoffset in dependence on a measurement of signal quality of at least oneof the plurality of radio frequency signals.
 17. A receiver according toclaim 11, wherein said signal processor is configured to: downconvertsaid plurality of radio frequency signals using quadrature mixing togive inphase and quadrature components; filter said inphase andquadrature components using a first bandpass filter bandwidth to givefirst bandpass filtered inphase and quadrature components; and filtersaid inphase and quadrature components using a second bandpass filterbandwidth, different from the first bandpass filter bandwidth, to givesecond bandpass filtered inphase and quadrature components.
 18. Areceiver according to claim 17, the receiver comprising at least one of:a first complex filter configured to perform said filtering of saidinphase and quadrature components to give first bandpass filteredinphase and quadrature components using; and a second complex filterconfigured to perform said filtering of said inphase and quadraturecomponents to give second bandpass filtered inphase and quadraturecomponents using.
 19. A receiver according to claim 17, the receiverbeing configured to: downconvert the first subset of the downconvertedplurality radio frequency signals from radio frequency bands in thefirst group; downconvert the second subset of the downconvertedplurality radio frequency signals from radio frequency bands in thesecond group; receive a first subset of the downconverted plurality ofradio frequency signals using the first bandpass filtered inphase andquadrature components; and receive a second subset of the downconvertedplurality of radio frequency signals using the second bandpass filteredinphase and quadrature components.
 20. A receiver according to claim 19,wherein the receiver further comprises: a plurality of analogue todigital converters configured to digitise the respective bandpassfiltered inphase and quadrature components; and at least a digital imagereject mixer to downconvert the digitised respective bandpass filteredinphase and quadrature components.